Relation of Soils to Moisture and Air 53 



Ground Plans In cutting ditches for imderdrains they 



and Grading should run in a direct Hne to a general 

 of Ditches , . , ^- 1 r 1 ? 



drain or outlet. The grade of the bottom 



of the ditch must be made perfectly uniform, since any 



break in the grade will form sags that retain silt and 



choke the drains. In a piece of low ground bordered by 



high land and with a stream of water running through, 



the drainage is simple, as the ditches should run from the 



base of the hills directly to the stream, incHning somewhat 



in the direction of the stream to prevent any choking of 



the outlet. In this case the ground plan of the drains will 



be Uke the bones of a fish with the stream representing the 



backbone. The ditches should be as narrow as they can 



well be dug so that the tile at the bottom will rest in a sohd 



bed of similar size and shape to the tile used. 



The distance between the Hne of ditches will vary with 

 local conditions. Where the fall is sHght and the ditches 

 not very deep, they will need to be nearer together; even 

 where there is a good fall and the ditches are cut deeply 

 they will be better near together, if there are strong spring 

 heads near the higher ground. In fact we have found it 

 an advantage to run a line of large tile around the base of 

 the hill to cut off the springs and then to branch the tiles 

 from this line to the outlet. 



The size of the tile used will depend on the amount of 

 water to be carried. Where the wetness of the land is 

 caused by strong springs that break out and overflow it, 

 there will be need of larger tiles to carry off this water than 

 where the wetness is simply due to a general nearness of 

 the water-table to the surface. Where the drains empty 

 into an open ditch or stream the outlet must be protected 



